As use of the video display terminal ("VDT") has become more widespread, for example in connection with computers, so too have certain ophthalmological problems associated with its use become more common. A recent survey of optometrists, reported in the J. Am. Optore. Assoc. 1992 (vol. 63, pp 687-92), shows that more than 14% of optometric patients present with symptoms primarily associated with use of the VDT, or almost 10 million examinations annually when projected to the U.S. population. Responding optometrists were unable to confidently arrive at a diagnosis and treatment more frequently for VDT patients (20.87%) compared to non-VDT patients (14.05%).
Alphanumeric characters displayed on video display screens are made up of dots or "pixels" which do not have well-defined edges and therefore are difficult for the eye to focus upon. Further, since video screens are maintained at a constant distance of about 50 cm from the user's eyes, the same eye muscles are in constant use in focusing on the screens. These factors cause significant amounts of stress and fatigue on the eyes of VDT users which are often aggravated by the fact that many such users utilize their computers for extended periods on a daily basis. The stress associated with video display use frequently results in peculiar types of eye problems requiring special corrective prescriptions in the spectacles selected for the users suffering from these problems.
In order to accurately diagnose these problems, appropriate test equipment and test procedures must be provided. In accordance with the process currently used by medical practitioners to determine the spectacle requirements of typical patients, an apparatus (phoropter) is placed in front of the eyes of the patient which enables the doctor to rapidly change a wide selection of lenses while the patient views a set of test images through the lens changing apparatus. As the patient focuses on the test images, the doctor assesses the status of the muscles inside the patient's eyes and judges their degree of relaxation through the use of a retinoscope. The doctor determines the combination of lenses and the prescription best suited to the patient by changing the lenses until he detects the combination which provides the most relaxed state in the eye muscles of the patient.
As may be understood from the above, the fitting of corrective lenses is basically a trial and error process in which the doctor observes the reaction of the patient's eye muscles to an appropriate test image for various combinations of lenses. However, without a test image which accurately simulates the conditions under which the patient may experience eye problems, a prescription for suitable corrective lenses may not be reliably determined. When presented with an image which does not have sharply defined edges or which is slightly out of focus, the eye will respond by reverting to a level of tonic muscle activity known as the resting point of accommodation, having a focal length with a sharply defined image placed at the same distance from the eye. Accordingly, the prescription required for eyeglasses used with a video display terminal can differ from a prescription for use in viewing printed material.
A vision tester apparatus is disclosed in commonly-owned U.S. Pat. No. 5,191,367. That apparatus, although quite useful for its intended purpose--computer screen simulation for optometric examinations--is costly to manufacture. It also is susceptible to various mechanical or electrical failures. The need remains, therefore, for providing eye care practitioners with equipment for VDT-user examinations that is inexpensive, reliable and easy to maintain.